There is generally known a vacuum pump having a casing secured to a driving machine such as an electrically-operated motor or the like and a rotational compression element rotated by the driving machine in a cylinder chamber of the casing. In this type vacuum pump, the rotational compression element is driven in the cylinder chamber by the driving machine to obtain vacuum. For example, the vacuum pump is mounted in an engine room of a vehicle and used to generate vacuum for actuating a brake booster (see Patent Document 1, for example).